This invention relates to a method of controlling an exhaust gas recirculating valve in an internal combustion engine, and more particularly to a method of this kind, which is adapted to prevent making a wrong diagnosis upon detecting abnormality in a valve opening control system for the exhaust gas recirculating valve, including a sensor for detecting the valve opening thereof.
An exhaust gas recirculation method is generally known, which is adapted to recirculate part of exhaust gases from the exhaust passage of an internal combustion engine to the intake passage of the engine to thereby reduce nitrogen oxide (NOx) which is one of the detrimental gases generated from the engine. Also, in order to determine an appropriate exhaust gas recirculating quantity in response to operating conditions of the engine, an exhaust gas recirculating valve control method has been used, which is adapted to detect actual valve opening of an exhaust gas recirculating valve arranged across an exhaust gas recirculating passageway, and control the actual valve opening value so that it approaches a desired valve opening value set in response to operating conditions of the engine.
In such an exhaust gas recirculating valve control method as stated above, when abnormality occurs in a valve opening control system including a valve opening detecting sensor for detecting the valve opening of the exhaust gas recirculating valve, due to disconnection in the wiring, etc., it is impossible to accurately control the exhaust gas recirculating quantity, which can even cause the phenomenon that exhaust gas recirculation takes place even when it is not required, depending upon operating conditions of the engine, resulting in heavy deterioration of the engine driveability.
Therefore, it has been known to give an alarm when it is detected that the difference or the ratio between the actual valve opening value detected by the valve opening detecting sensor and the desired valve opening value falls outside an allowable range (Japanese Provisional Patent Publication (Kokai) No. 55-123345). However, if the desired valve opening value calculated in response to operating conditions of the engine exceeds a maximum possible valve opening of the exhaust gas recirculating valve, i.e., a valve opening beyond which the exhaust gas recirculating valve cannot be operated, the difference between the actual valve opening value and the desired valve opening value will not be reduced even if a control signal responsive to the difference is issued for operating the exhaust gas recirculating valve to the desired valve opening value. As a consequence, it can be wrongly judged that the exhaust gas recirculating valve has developed an abnormality. A situation similar to the above can occur if the desired valve opening value calculated in response to operating conditions of the engine exceeds a minium possible valve opening of the exhaust gas recirculating valve, resulting in a wrong judgement that the exhaust gas recirculating has developed an abnormality.